map_flatten: make it a rustfix test

tests/ui/map_flatten.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+
+#![warn(clippy::all, clippy::pedantic)]
+#![allow(clippy::missing_docs_in_private_items)]
+
+fn main() {
+    let _: Vec<_> = vec![5_i8; 6].into_iter().flat_map(|x| 0..x).collect();
+}

tests/ui/map_flatten.rs

@@ -1,3 +1,5 @@
+// run-rustfix
+
 #![warn(clippy::all, clippy::pedantic)]
 #![allow(clippy::missing_docs_in_private_items)]
 

tests/ui/map_flatten.stderr

@@ -1,5 +1,5 @@
 error: called `map(..).flatten()` on an `Iterator`. This is more succinctly expressed by calling `.flat_map(..)`
-  --> $DIR/map_flatten.rs:5:21
+  --> $DIR/map_flatten.rs:7:21
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using flat_map instead: `vec![5_i8; 6].into_iter().flat_map(|x| 0..x)`